Ammunition cartridge cases have been manufactured in the past in accordance with various methods and from various types of materials such as brass, aluminum and even steel. Many of these prior art methods require a series of drawing and ironing steps, or have required complicated machines for fabricating the cartridge cases. However, despite the continued work which has gone on in this art and the suggestion of numerous methods and apparatus, prior art methods and apparatus have not proved completely satisfactory in economically fabricating high quality cartridge cases.
Initially, in this regard, as used herein, the term "drawing" is used in its normal sense in referring to the operation wherein a peripheral margin of a flat blank is turned upwardly and simultaneously smoothed by means of a drawing punch and die to form a cup having a wrinkle-free side wall whose thickness is substantially equal to the thickness of the original blank. Subsequent "redrawing" of the cup merely turns up more of the end material into the side wall (without any substantial reduction in wall thickness), thereby elongating the side wall as a result of a substantial reduction in the diameter of the cup.
The term "ironing" is also used in its conventional sense in referring to the operation wherein the side wall of the cup is elongated by reducing its thickness with no appreciable reduction in the inside diameter of the cup. It is generally accomplished by placing the cup on a closely fitting punch or mandrel and forcing the cup and mandrel through an ironing or reducing die whose diameter is slightly less than the outer diameter of the cup, thereby forcing the excess metal back and producing a longer but thinner side wall.
In this regard, it is initially to be noted that special ballastic considerations are involved in the manufacture of cartridge cases. For example, it is desired that cartridge cases be lightweight while at the same time be manufactured from materials having high strength and tear resistance to ensure against jamming or tearing during the firing operation. Generally, much of this strength and toughness is imparted to the material from which the cartridge case is manufactured by virtue of the working of the material during the shaping and forming operations. This is particularly true with respect to brass materials (i.e., copper and zinc alloys). Additionally, in order to prevent fracture of the side wall of the case during firing, cartridge cases generally include an inner side wall surface which tapers outwardly from its base toward its opposite open end (i.e., the side wall thickness decreases along the length of the cartridge case from the base or closed end toward the open end). Still further, in order to provide a highly concentric shell in order to provide desired firing characteristics for the ammunition round, it is most important to maintain the concentricity of the cartridge case (i.e., minimize the variation in wall thickness about the circumference of the formed cartridge case). Concentricity is also important in order to prevent splitting or fracturing of the case during firing. A further ballastic consideration is reloadability, i.e., the capability of utilizing a cartridge case for a number of firings.
Many typical prior art methods of manufacturing catridge cases have involved stamping out circular discs from precision sheeting, and then drawing the disc into a cup-shaped article. Thereafter, the cup-shaped aritcle is forced through a series of drawing and ironing dies to produce the finished cartridge case. The preformed cup-shaped article generally has an outer and inner diameter which is larger than the finished inner and outer diameters of the formed cartridge case. Thus, the series of drawing and ironing operations serve to reduce not only the outside diameter but also the inside diameter, and additionally to reduce the wall thickness of the side walls of the cartridge case from the thickness of the sheeting from which the blank was stamped.
In this regard, because the metal becomes hardened when worked, it has been necessary in the past to interrupt the passing of the article through the die members and to anneal the intermediately formed article before passing it through subsequent dies and thus complete the forming of the article into the finished desired cartridge case. This is particularly true in the case of cartridge cases made from brass.
More specifically, in the past it has generally been necessary to subject the cup-shaped article to several separate annealing operations to recrystallize the elongated grains of the metal before completion of the cartridge case--i.e., after the article has been partially processed and before being passed through subsequent die members. As an example, in some processes, after the disc is formed into a cup-shaped article, an initial annealing operation is required. The annealed cup can then be forced through a first series of dies, employing a first punch element having a smaller diameter than the inside diameter of the cup-shaped article. During this operation, the article shrinks onto the first punch element (i.e., is redrawn) and its wall thickness is also reduced. Thereafter, the article must again be subjected to an annealing operation in order to relieve stresses created by working the metal in its passage through the dies. The cup is then subjected to a second redrawing and ironing operation, a third annealing step, and a final redrawing and ironing operation. It will thus be appreciated that in accordance with these prior art methods the cup-shaped article is subjected to both a number of drawing or redrawing operations (i.e., in which the inner and outer diameters of the cup-shaped article are reduced without a significant reduction in wall thickness) and to a number of ironing operations (i.e., in which only the outer diameter is reduced, to thereby reduce the thickness of the wall). Furthermore, because of the number of these drawing and ironing operations and the concomitant amount of working which takes place, several intermediate annealing operations are required in order to relieve the stresses built up in the metal during each of these series of drawing and redrawing operations.
Those skilled in the art have also appreciated that each of these annealing operations for recrystallizing the elongated grains require a considered expenditure of energy in the form of heat (either electrical energy or combustion energy) as well as substantial amounts of material and labor cost in connection with heating and treating the cup-shaped article prior to further passage through subsequent dies. Still further, in these prior art methods large amounts of complex tooling are required (i.e., separate punch elements for each series of dies, as well as a number of die members for each drawing and ironing operation). Significant expenses are also incurred in using precision sheeting from which the disc is blanked. Furthermore, the use of sheeting results in significant amounts of waste by virtue of the skeleton which remains after the discs have been stamped therefrom. Each of these considerations greatly serves to increase the cost of manufacture of conventional cartridge cases.
It has also been suggested in the past to manufacture the initial cup-shaped article from wire stock in which a slug of material is cut and then subjected to a cold heading operation to extrude the slug into a cup-shaped article. (See for example U.S. Pat. Nos. 2,028,996 and 2,371,716.) However, even in the methods disclosed in these patents, it is still necessary to subject the cup-shaped article to a series of drawing operations which reduce the inside diameter of the article, as well as ironing operations intended to produce the final desired shape of the cartridge case. After each of the drawing operations, it remains necessary to anneal the article before subsequent drawing and ironing operations are performed.
Single step formation by simply drawing a flat sheet of metal into a cup-shaped article and then only ironing the cup-shaped article into a finished product has not been applied to the art of cartridge manufacture in the past. On the other hand, such formation has been utilized in the manufacture of cylindrical seamless containers (see, for example, British Patent Specification No. 625,011; U.S. Pat. No. 2,412,813; and U.S. Pat. No. 3,203,218). However, such prior art methods for forming metal seamless containers would not generally have been considered to be applicable to the manufacture of cartridge cases, particularly in view of the high precision and special shapes and configurations involved in cartridge case manufacture. For example, while prior art container manufacturing techniques have utilized very precise dies having precisely determined diameters, entrance angles and exit angles, the containers which are manufactured all have a constant wall thickness along substantially their length (generally the mouth of the container may be somewhat thicker for forming a seam with the lid). This is in contrast to cartridge cases, which generally include a tapered wall surface which tapers from the base end of the cup-shaped cartridge to the open formed end of the cartridge case. Also, such methods have not previously been applied with respect to containers having large length to diameter ratios such as exist with cartridge cases.
While it has also been suggested in the prior art to attempt to manufacture the cartridge case in a single continuous stroke of a punch element (see for example U.S. Pat. Nos. 2,140,775; 3,977,225; and 4,129,024), each of these prior art methods have required complicated, and therefore expensive, machinery, such as for example coaxially movable punch members, cushions for the dies, etc. These prior art methods have not employed preformed cup-shaped articles which are then only ironed to elongate the side walls while reducing the thickness thereof. Further, generally the punch element which is used to form the cup-shaped article is tapered so that the finished cartridge case will have an inner side wall surface which is tapered.
Consequently, it will be appreciated that a need exists with respect to the manufacture of ammunition cartridge cases for a more economical method of manufacture which still produces a precision cartridge case having good concentricity and with a tapering wall section along its longitudinal length.